1. Field of the Invention
This invention relates to servo-accelerometers suited to be used as a high precision vibration detector for instruments subject to vibrations of relatively low frequency and, more particularly, to servo-accelerometers which, when used in cameras or like instruments, detect vibrations from, for example, hand shake to permit an optimum performance of the image blur preventing system thereof.
2. Description of the Related Art
As is well known, the frequencies of, for example, the normal jiggle of a camera are distributed in a low region of from 1 to 12 Hz. In order to make up an image blur preventing system which can militate against such delicate vibrations, a high precision acceleration sensor must be employed. Such an accelerometer is disclosed in U.S. Pat. No. 4,611,491.
A production view of one prior known servo-accelerometer is shown in FIG. 3, where a housing of the servo-accelerometer is comprised of a housing lid 105 and a housing bottom 101. The housing lid 105 is fixedly secured to the housing bottom 101. A leaf-shaped support spring 102 of small rigidity is supported at its ends by the housing lid 105 and the housing bottom 101, respectively. By means of the spring 102, a pendulum 104 on which a coil 103 is mounted is swingably suspended. To unify the pendulum 104 with the support spring 102, the pendulum 104 is provided with jaw members 104a and 104b for clamping the support spring 102. It should be noted that, to mount the support spring 102 in a fixedly secured relation to the housing, its upper end is sandwiched between abutments 101b and 105a of a housing side 101a and the housing lid 105 respectively, and its lower end between abutments 101c and 101d of the housing side 101a and the housing bottom 101 respectively.
Above and below the assembly of the coil 103 and the pendulum 104, there is a magnetic circuit plate in the form of the lid 105 and two pair of permanent magnet pieces 106a and 106b, and 106c and 106d, respectively, in spaced relation to the assembly. The upper magnetic circuit plate also serves as the housing lid 105 as has been described above. The permanent magnet pieces 106a and 106b are fixedly mounted on a magnetic circuit back plate 107 which is secured to the housing bottom 101.
At the central area of the pendulum 104 there is provided a slit 108 through the wall thereof. An infrared light-emitting diode or light projector 109 is arranged on the inner surface of the housing lid 105 in alignment with the slit 108. Beneath the slit 108 is arranged a position sensitive device (PSD), silicon photo-cell (SPC) or like photo-electric type displacement detector 110 on the inner surface of the magnetic circuit back plate 107. Stoppers 115 and 116 limit a range of swinging movement of the pendulum 104. By this pendular amplitude limit means or stoppers 115 and 116, a larger torsional force than necessary is prevented from acting on the support spring 102.
Now assuming that an acceleration "a" works on the housing in a direction indicated by the arrow in FIG. 3, then the pendulum 104 is related to swing in the opposite direction to the acceleration "a". This pendular angle can be detected by the displacement detector 110 on the basis of a position of a light beam emitted from the projector 109 through the slit 108 to the displacement detector 110.
Incidently, a magnetic flux from the permanent magnet piece 106a goes in the following direction: the permanent magnet piece 106a.fwdarw.coil 103.fwdarw.magnetic circuit plate (105).fwdarw.coil 103.fwdarw.permanent magnet piece 106b. Another magnetic flux from the permanent magnet piece 106b has the following route: the permanent magnet piece 106b .fwdarw.magnetic circuit back plate 107.fwdarw.permanent magnet piece 106a. In all, a closed magnetic circuit is formed with the magnetic fluxes in perpendicular directions to the coil 103. By a control current flowing to the coil 103, according to Fleming's rule, the pendulum 104 can be moved to either side in the direction of displacement in which the acceleration "a" works.
FIG. 4 shows an example of the structure of the acceleration detecting circuit usable in the accelerometer of the character described above. This circuit comprises the above-described displacement detector 110, an amplifier 111 for amplifying an output of the detector 110, a compensation circuit 112 for stabilizing this feedback circuit, a transistor circuit 113 for further amplifying the output of the compensation circuit 112, and the coil 103 arranged in a common series connection. And, in this example, the direction of winding of the coil 103 and the orientation of the polarities of the permanent magnet pieces 106a and 106b are so determined that when the current flows to the coil 103, the direction of the exerted force is opposite to the pendular direction of the pendulum 104 by the external acceleration "a".
The operative principle of the servo-accelerometer of such construction is explained below. Now assuming that the accelerometer is given the acceleration "a" from the outside of its housing, then the pendulum 104 apparently swings by the inertial force in the opposite direction to that of the displacement of the housing. Therefore, the slit 108 of the pendulum 104 moves in a direction L in FIG. 4, causing a change of the position of incidence of the center of the light beam from the projector 109 to the displacement detector 110. Hence, the output of the displacement detector 110 is proportional to the displacement amount of the pendulum 104.
This output is then processed by the circuit of FIG. 4 in the manner described above. In more detail, it is amplified by the displacement detecting amplifier 111, and further current amplification is carried out by the transistor circuit 113 to energize the coil 103.
With this, the direction of current flow to the coil 103 is controlled in such a manner that a force of the opposite direction R to the direction L of displacement of the housing, or acceleration "a", is exerted on the pendulum 104. Thus, the point of incidence of the light beam on the displacement detector 110 tends to return to the initial position which it took when the acceleration "a" was not applied to the housing.
Further, the intensity of this control current flowing through the coil 103 is proportional to the magnitude of the force of returning the pendulum 104 to the original point, which in turn is proportional to the force applied to the pendulum 104, in other words, the magnitude of acceleration "a". Hence, by using a resistor 114 to read the current in the form of a voltage V, it is made possible to obtain the magnitude of acceleration "a" as information necessary to control the operation of the image blur preventing system in, for example, the camera.
As that the requirement for acceleration detection be performed with high accuracy , such a servo-accelerometer has been found to be sufficient. From the point of view of its constructional features, on the other hand, the requirement for the precision accuracy of assembly is very rigorous. Therefore, the above-described accelerometer has a problem that the efficiency of assembling operation is difficult to increase.
Taking an example of the fitting relationship of the coil 103 on the pendulum 104 as shown in FIG. 5, because the coil 103 is generally formed to a number of windings of an enameled wire, its size differs relatively largely from item to item. Therefore, the design size of that area 104c of the pendulum 104 on which the coil 103 is to be mounted (for example, the recessed portion or opening in which it is to be fitted) must be taken at so large a value as to enable mounting of the coils of the maximum possible size. For smaller coils, on the other hand, it becomes difficult to accurately regulate their attitude. Therefore, it will result that the coil 103 is mounted obliquely relative to the plane of the pendulum 104.
And, if such oblique mounting takes place, there will be a serious loss of the detection accuracy of the accelerometer.
Another requirement is that the lead wires to the coil 103 be provided on the pendulum 104 without preventing the movement of the pendulum 104. To fulfill this, it has been usual in the prior art that a pair of terminals are positioned near the support spring 102 to interconnect the coil 103 to the circuit of FIG. 4. The use of such means increases the difficulty of the assembling operation. Another problem is that as the size of the solder spot on the connection between either end of the coil 103 and the terminal increases, there is some possibility of interference with the permanent magnet piece 106a, 106b, or the lid 105 as the magnetic circuit plate.
Yet another problem arises from the necessity of giving a proper dynamic range for the pendulum 104 as is suggested by the use of the stoppers described above. If this pendular amplitude limit means is formed to such stoppers and positioned at such locations as indicated at 115 and 116 in FIG. 3, as was usual in the past, because the framework as the base of the stopper is too complicated in structure to be made up as a unit with the stoppers using a molding technique, that the amplitude limit means cannot be produced without involving a problem. As to the material of the stoppers, in order not to disturb the magnetic field, use of a non-magnetic material is preferred. In the framework, on the other hand, for the formation of the magnetic circuit, the use of magnetic material prevails. For these reasons, it is substantially difficult to form the stoppers as a unified part to the framework. In the prior art, therefore, an additional problem arose that the stoppers had, after all, to be prepared as the separate parts and later assembled with the framework, or such troublesome and time-consuming steps had to be introduced into the process of manufacture.